Variable refrigerant charge control

ABSTRACT

An apparatus and method for adjusting refrigerant charge level are provided. The apparatus has a reservoir, a reservoir line, a reservoir valve, and one or more side valves. The reservoir line connects the reservoir and a liquid line, and has a connection to the liquid line. The liquid line connects an indoor heat exchanger and an outdoor heat exchanger. The reservoir valve is on the reservoir line. The one or more side valves are on the liquid line. In the method, an indicator of effectiveness of a refrigerant-using system is calculated. The indicator is compared to a target indicator of effectiveness. A refrigerant charge level is adjusted to reduce the difference between the indicator and the target indicator.

TECHNICAL FIELD

This application relates to HVAC systems and, more particularly, to HVACrefrigerant charge levels.

BACKGROUND

One area that has not been fully optimized in Heating, Ventilation, andAir Conditioning (HVAC) systems is the refrigerant charge level.Variable speed compressor technology greatly increased the efficiency ofHVAC systems by allowing the compressor speed to be better adjusted tomatch the load on the system. However, the refrigerant charge level(amount of refrigerant in the system) in a conventional HVAC systemremains the same regardless of the load on the system. The refrigerantcharge level is therefore optimized for a single operating condition. Itwould be desirable if a HVAC system could optimize its refrigerantcharge level for the current operating condition.

SUMMARY

In an embodiment, an apparatus for adjusting refrigerant charge level isprovided. The apparatus has a reservoir, a reservoir line, a reservoirvalve, and one or more side valves. The reservoir line connects thereservoir and a liquid line, and has a connection to the liquid line.The liquid line connects an indoor heat exchanger and an outdoor heatexchanger. The reservoir valve is on the reservoir line. The one or moreside valves are on the liquid line.

In another embodiment, a method for adjusting refrigerant charge levelis provided. An indicator of effectiveness of a refrigerant-using systemis calculated. The indicator is compared to a target indicator ofeffectiveness. A refrigerant charge level is adjusted to reduce thedifference between the indicator and the target indicator.

DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following DetailedDescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 depicts a HVAC system with a refrigerant charge controlapparatus;

FIG. 2A depicts the refrigerant charge control apparatus configured fornormal operation;

FIG. 2B depicts the refrigerant charge control apparatus configured tofill a reservoir during cooling;

FIG. 2C depicts the refrigerant charge control apparatus configured tofill the reservoir during heating;

FIG. 2D depicts the refrigerant charge control apparatus configured todrain the reservoir using gravity;

FIG. 2E depicts the refrigerant charge control apparatus configured todrain during cooling;

FIG. 2F depicts the refrigerant charge control apparatus configured todrain during heating;

FIG. 3 depicts a HVAC system with an alternate refrigerant chargecontrol apparatus;

FIG. 4A depicts the alternate refrigerant charge control apparatusconfigured for normal operation;

FIG. 4B depicts the alternate refrigerant charge control apparatusconfigured to fill a reservoir;

FIG. 4C depicts the alternate refrigerant charge control apparatusconfigured to drain the reservoir during cooling;

FIG. 4D depicts the alternate refrigerant charge control apparatusconfigured to drain the reservoir during heating;

FIG. 5 depicts a method which a controller may perform to use asubcooling value to control the refrigerant charge; and

FIG. 6 depicts a method which a controller may perform to use an EnergyEfficiency Ratio (EER) to control the refrigerant charge.

DETAILED DESCRIPTION

In the following discussion, numerous specific details are set forth toprovide a thorough explanation. However, such specific details are notessential. In other instances, well-known elements have been illustratedin schematic or block diagram form. Additionally, for the most part,specific details within the understanding of persons of ordinary skillin the relevant art have been omitted.

With reference to FIG. 1, depicted is a Heating, Ventilation, and AirConditioning (HVAC) system 100 with a refrigerant charge controlapparatus 101. System 100 includes indoor unit 102, outdoor unit 104,and controller 105. Indoor unit 102 would be located inside a structureto be heated or cooled, such as a building or refrigerator. Outdoor unit104 would be located outside the structure. This combination of anindoor unit and an outdoor unit is generally used in residential HVACsystems but may also be used in other applications, such asrefrigeration.

Prior to the operation of apparatus 101, HVAC system 100 operatesconventionally. A continuous flow of refrigerant moves in a loop throughHVAC system 100. This loop may be called the “vapor compression cycle.”Compressor 106 compresses refrigerant in gas vapor form, and thendischarges the refrigerant through discharge line 108. The compressedrefrigerant gas vapor enters reversing valve 110. Reversing valve 110can change between a cooling configuration, shown by solid lines, and aheating configuration, shown by dashed lines.

In the cooling configuration, the refrigerant flows from reversing valve110 to outdoor heat exchanger 112. The refrigerant flows through outdoorheat exchanger 112, releasing heat into the outdoor air and condensinginto a liquid. From outdoor heat exchanger 112, the liquid refrigerantflows through liquid line 114.

Liquid line 114 has expansion device 116A and expansion device 116B.Expansion devices 116A and 116B expand liquid refrigerant flowingthrough them, reducing the pressure of the refrigerant. However, due tocheck valves or the like, expansion device 116A only acts on refrigerantflowing toward outdoor heat exchanger 112, and expansion device 116Bonly acts on refrigerant flowing toward indoor heat exchanger 118.Refrigerant flowing in the opposite directions, through expansion device116A toward indoor heat exchanger 118 or through expansion device 116Btoward outdoor heat exchanger 112, bypasses the respective expansiondevice and does not expand.

The liquid refrigerant bypasses expansion device 116A and flows toexpansion device 116B. Expansion device 116B reduces the pressure of theliquid refrigerant flowing through it. The refrigerant then flowsthrough indoor heat exchanger 118, absorbing heat from the structure andevaporating into a gas vapor. The refrigerant then flows to reversingvalve 110, where it is directed through suction line 120 and back intocompressor 106 to be compressed again.

In the heating configuration, the refrigerant flows from reversing valve110 to indoor heat exchanger 118. The refrigerant flows through indoorheat exchanger 118, releasing heat into the structure and condensinginto a liquid. From indoor heat exchanger 118, the liquid refrigerantflows through liquid line 114. The liquid refrigerant bypasses expansiondevice 116B and flows to expansion device 116A. Expansion device 116Areduces the pressure of the liquid refrigerant flowing through it. Therefrigerant then flows through outdoor heat exchanger 112, absorbingheat from the outdoor air and evaporating into a gas vapor. Therefrigerant then flows to reversing valve 110, where it is directedthrough suction line 120 and back into compressor 106 to be compressedagain.

Outdoor heat exchanger 112 may be called the outdoor coil. Indoor heatexchanger 118 may be called the indoor coil. During cooling, outdoorheat exchanger 112 may be called the condenser and indoor heat exchanger118 may be called the evaporator. During heating, outdoor heat exchanger112 may be called the evaporator and indoor heat exchanger 118 may becalled the condenser. Expansion devices 116A and 116B may be expansionvalves.

Refrigerant charge control apparatus 101 comprises reservoir line 124,reservoir 126, reservoir valve 128A, indoor side valve 128B, and outdoorside valve 128C. Reservoir line 124 connects liquid line 114 toreservoir 126. Reservoir 126 may be a tank which holds excessrefrigerant.

Reservoir valve 128A may be positioned on reservoir line 124. Indoorside valve 128B may be positioned on liquid line 114 between reservoirline 124 and indoor heat exchanger 118. Outdoor side valve 128C may bepositioned on liquid line 114 between reservoir line 124 and outdoorheat exchanger 112. Valves 128A, 128B, and 128C can each be opened, topermit the flow of refrigerant, or closed, to block the flow ofrefrigerant. Valves 128A, 128B, and 128C may be solenoid valves.

Indoor side valve 128B and outdoor side valve 128C are called “indoor”and “outdoor” to identify their locations relative to reservoir line 124and heat exchangers 112 and 118. The “indoor” and “outdoor” names do notidentify whether the valves 128B-C are indoors or outdoors. Indoor sidevalve 128B may be located indoors or outdoors. Outdoor side valve 128Cmay be located indoors or outdoors.

Refrigerant charge control apparatus 101 can be operated to fillreservoir 126 with refrigerant from liquid line 114, reducing the amountof refrigerant for compressor 106 to compress. Refrigerant chargecontrol apparatus 101 can also be operated to drain refrigerant fromreservoir 126 into liquid line 114, increasing the amount of refrigerantfor compressor 106 to compress.

Controller 105 operates valves 128A, 128B, and 128C to adjust the“refrigerant charge level,” the amount of refrigerant in the vaporcompression cycle. Where valves 128A-C are solenoid valves, controller105 may send current through valves 128A-C directly or send a signalthat causes current to be sent through valves 128A-C. Controller 105 maybe a unit controller that controls the overall operation of units 102and 104, or may be a separate controller that only controls therefrigerant charge level.

With reference to FIG. 2A, depicted is a configuration 200A ofrefrigerant charge control apparatus 101 in normal operation, whenreservoir 126 is not being drained or filled. Reservoir valve 128A isclosed, indoor side valve 128B is open, and outdoor side valve 128C isopen. Refrigerant flows through liquid line 114 as it would in theabsence of refrigerant charge control apparatus 101. In FIG. 2A,refrigerant would flow through liquid line 114 from left to right duringcooling and from right to left during heating.

With reference to FIG. 2B, depicted is a configuration 200B ofrefrigerant charge control apparatus 101. In configuration 200B,refrigerant charge control apparatus 101 is configured to fill reservoir126 during cooling. Reservoir valve 128A and outdoor side valve 128C areopen, while indoor side valve 128B is closed. Refrigerant 202 flowingfrom outdoor heat exchanger 112 through liquid line 114 is blocked byindoor side valve 128B. Refrigerant 202 is instead forced throughreservoir line 124 into reservoir 126. After charge is added toreservoir 126, refrigerant charge control apparatus 101 may return toconfiguration 200A.

With reference to FIG. 2C, depicted is a configuration 200C ofrefrigerant charge control apparatus 101. In configuration 200C,refrigerant charge control apparatus 101 is configured to fill reservoir126 during heating. Reservoir valve 128A and indoor side valve 128B areopen, while outdoor side valve 128C is closed. Refrigerant 202 flowingfrom indoor heat exchanger 118 through liquid line 114 is blocked byoutdoor side valve 128C. Refrigerant 202 is instead forced throughreservoir line 124 into reservoir 126. After charge is added toreservoir 126, refrigerant charge control apparatus 101 may return toconfiguration 200A.

With reference to FIG. 2D, depicted is a configuration 200D ofrefrigerant charge control apparatus 101. In configuration 200D,refrigerant charge control apparatus 101 is configured to drainreservoir 126 using gravity. Indoor side valve 128B and outdoor sidevalve 128C are open, allowing refrigerant 202 to flow through liquidline 114 normally. Reservoir valve 128A is also open, allowing gravityto drain refrigerant 202 in reservoir 126 into liquid line 114. In FIG.2D, refrigerant would flow through liquid line 114 from left to rightduring cooling and from right to left during heating. After charge isremoved from reservoir 126, refrigerant charge control apparatus 101 mayreturn to configuration 200A.

Because configuration 200D depends on gravity, to use configuration 200Dreservoir 126 should be placed above liquid line 114. As an alternativeto configuration 200D, configurations 200B and 200C can be used to drainreservoir 126 using a pressure difference. Reservoir 126 may thereforebe placed at the same height as or lower than liquid line 114. Ifreservoir 126 is above liquid line 114, gravity can still aidconfigurations 200B and 200C in draining reservoir 126.

Referring to FIG. 2E, depicted is configuration 200C used to drainreservoir 126 during cooling. Outdoor side valve 128C is closed,blocking the flow of refrigerant from outdoor heat exchanger 112 andreducing the pressure on the other side of outdoor side valve 128C.Reservoir valve 128A and indoor side valve 128B are open. The reducedpressure draws refrigerant from reservoir 126 into liquid line 114.After charge is removed from reservoir 126, refrigerant charge controlapparatus 101 may return to configuration 200A.

Referring to FIG. 2F, depicted is configuration 200B used to drainreservoir 126 during heating. Indoor side valve 128B is closed, blockingthe flow of refrigerant from indoor heat exchanger 118 and reducing thepressure on the other side of indoor side valve 128B. Reservoir valve128A and outdoor side valve 128C are open. The reduced pressure drawsrefrigerant from reservoir 126 into liquid line 114. After charge isremoved from reservoir 126, refrigerant charge control apparatus 101 mayreturn to configuration 200A.

With reference to FIG. 3, depicted is a Heating, Ventilation, and AirConditioning (HVAC) system 300 with an alternate refrigerant chargecontrol apparatus 301. System 300 is identical to system 100 except thatapparatus 301 has been substituted for apparatus 101. Refrigerant chargecontrol apparatus 301 comprises reservoir line 124, reservoir 302,reservoir valve 128A, indoor side valve 128B, and outdoor side valve128C. Reservoir line 124 may connect liquid line 114 to reservoir 302.Valves 128A, 128B, and 128C may be positioned as in apparatus 101.Controller 105 operates valves 128A, 128B, and 128C to adjust therefrigerant charge level.

Reservoir 302 may be a tank which holds excess refrigerant. Suction line120 passes through reservoir 302, and may pass through the middle ofreservoir 302. Refrigerant stored in reservoir 302 does not flow throughsuction line 120 into compressor 106. A tank with a suction line passingthrough it is commonly called a charge compensator.

With reference to FIG. 4A, depicted is a configuration 400A ofrefrigerant charge control apparatus 301 in normal operation, whenreservoir 302 is not being drained or filled. Reservoir valve 128A isclosed, indoor side valve 128B is open, and outdoor side valve 128C isopen. Refrigerant flows through liquid line 114 as it would in theabsence of refrigerant charge control apparatus 301. In FIG. 4A,refrigerant would flow through liquid line 114 from left to right duringcooling and from right to left during heating.

With reference to FIG. 4B, depicted is a configuration 400B ofrefrigerant charge control apparatus 301. In configuration 400B,refrigerant charge control apparatus 301 is configured to fill reservoir302. Reservoir valve 128A, indoor side valve 128B, and outdoor sidevalve 128C are open. The refrigerant passing through suction line 120 iscooler than the refrigerant passing through liquid line 114. Thetemperature difference draws refrigerant from liquid line 114 throughreservoir line 124 and into reservoir 302. After charge is added toreservoir 302, refrigerant charge control apparatus 301 may return toconfiguration 400A.

With reference to FIG. 4C, depicted is a configuration 400C ofrefrigerant charge control apparatus 301. In configuration 400C,refrigerant charge control apparatus 301 is configured to drainreservoir 302 during cooling. Reservoir valve 128A and indoor side valve128B are open, while outdoor side valve 128C is closed. The closedoutdoor side valve 128C blocks the flow of refrigerant through liquidline 114, reducing the pressure in liquid line 114 after valve 128Cbelow the pressure in suction line 120. Refrigerant drains fromreservoir 302 into liquid line 114 and flows toward indoor heatexchanger 118. After charge is removed from reservoir 302, refrigerantcharge control apparatus 301 may return to configuration 400A.

With reference to FIG. 4D, depicted is a configuration 400D ofrefrigerant charge control apparatus 301. In configuration 400D,refrigerant charge control apparatus 301 is configured to drainreservoir 302 during heating. Reservoir valve 128A and outdoor sidevalve 128C are open, while indoor side valve 128B is closed. The closedindoor side valve 128B blocks the flow of refrigerant through liquidline 114, reducing the pressure in liquid line 114 after valve 128Bbelow the pressure in suction line 120. Refrigerant drains fromreservoir 302 into liquid line 114 and flows toward outdoor heatexchanger 112. After charge is removed from reservoir 302, refrigerantcharge control apparatus 301 may return to configuration 400A.

HVAC systems 100 and 300 are capable of both heating and cooling. Asystem which can perform both may be called a heat pump. In a HVACsystem which is capable of one of heating or cooling, but not both, oneof valves 128B and 128C may be removed. In a HVAC system which is onlycapable of heating, also called a heater, indoor side valve 128B isunnecessary. In a HVAC system which is only capable of cooling, alsocalled an air conditioner, outdoor side valve 128C is unnecessary. Anexception is a refrigerant charge control apparatus 101 which relies onconfiguration 200B or 200C to drain reservoir 126. In such an apparatus101, both valves 128B and 128C are used even if the HVAC system is onlycapable of one of heating or cooling.

Additionally, in a heater or air conditioner, reversing valve 110 isunnecessary because the direction of refrigerant flow does not reverse.Expansion device 116A is also unnecessary in an air conditioner becauserefrigerant does not flow through liquid line 114 toward outdoor heatexchanger 112. Expansion device 116B is also unnecessary in a heaterbecause refrigerant does not flow through liquid line 114 toward indoorheat exchanger 118.

Refrigerant charge control apparatuses 101 and 301 are shown insideoutdoor unit 104. However, this is not necessarily the case. Refrigerantcharge control apparatuses 101 and 301 may also be inside indoor unit102.

Refrigerant charge control apparatuses 101 and 301 may fill or draintheir respective reservoirs by cycling between the normal operationconfiguration and a fill or drain configuration. For instance,refrigerant charge control apparatus 101 does not necessarily change toconfiguration 200B, wait for reservoir 126 to fill sufficiently, andthen change to configuration 200A. Refrigerant charge control apparatus101 could alternately begin cycling between configuration 200B andconfiguration 200A until reservoir 126 fills sufficiently, then changeto configuration 200A.

Depending on tubing size, using simple solenoid valves for valves 128A,128B, and 128C may result in refrigerant flow that is too fast. In anembodiment, valves 128A, 128B, and 128C are electronic flow valves withvariable flow rates. When an electronic flow valve 128A, 128B, or 128Cis opened, controller 105 may adjust the flow rate of the open valve toadjust the rate reservoir 126 or 302 fills or drains.

Compressor 106 is preferably a variable speed compressor, which canoperate at a wide range of possible speeds. Compressor 106 may also be amultiple stage compressor, which can operate at a few discrete speeds.Compressor 106 may also be a single stage compressor, which operates atonly a single speed. However, the benefit of adjusting the refrigerantcharge increases with the range of speeds compressor 106 is capable of.With a single stage compressor 106, the benefit is very limited. Thebenefit is also less with a multiple stage compressor 106 than avariable speed compressor 106.

With a variable speed or multiple stage compressor 106, the speed ofcompressor 106 increases when the load on the HVAC system is high anddecreases when the load on the HVAC system is low. Generally speaking,when there is a relatively low load on the HVAC system, the refrigerantcharge level should be relatively high. Ideally, only liquid refrigerantshould leave the expansion device which expands the refrigerant. Thisexpansion device is 116B in the cooling configuration and 116A in theheating configuration. If the refrigerant charge level is too low, amixture of liquid and gas refrigerant will leave the expansion device,which will reduce the performance of the evaporator coil.

Likewise, when there is a relatively high load on the HVAC system, therefrigerant charge level should be relatively low. Less refrigerant isneeded to keep gas refrigerant from leaving the expansion device whichexpands the refrigerant. At the same time, unnecessary refrigerantincreases the pressure of the refrigerant in the vapor compression cycleand additional power is used moving that excess refrigerant.

However, this inverse relationship between load and optimal refrigerantcharge level is only true in general. It is possible to have too high arefrigerant charge level with a low load or too low a refrigerant chargelevel with a high load. Thus, it is not necessarily possible todetermine whether the refrigerant charge level should be increased ordecreased solely from the present load on the HVAC system.

With reference to FIG. 5, depicted is a method 500 which controller 105may perform to control the refrigerant charge level. Method 500 uses asubcooling value to determine whether the refrigerant charge levelshould be changed. When the gas refrigerant passes through the condenserand changes into a liquid, the temperature of the refrigerant falls butthe refrigerant remains at the same pressure. The subcooling value isthe amount the temperature falls below the saturation temperature of therefrigerant for that pressure. The subcooling value is a measure of theeffectiveness of the system 100 or 300.

Controller 105 may have a memory which stores target subcooling valuesfor a given load on the HVAC system. The target subcooling valuesrepresent an ideal subcooling value when the refrigerant charge level isoptimized for a given load. These target subcooling values may bedetermined during testing or simulation of the HVAC system.

At 502, controller 105 may measure the temperature and pressure of theliquid leaving the condenser. At 504, controller 105 may calculate thesubcooling value from the temperature and pressure. At 506, controller105 may compare the subcooling value to the target subcooling value forthe present operating load.

At 508, controller 105 may operate valves 128A, 128B, and 128C on chargecontrol apparatus 101 or charge control apparatus 103 to adjust therefrigerant charge level. Whether to increase or decrease therefrigerant charge level may be a matter of trial and error forcontroller 105, based on whether the last adjustment to the refrigerantcharge level brought the subcooling value closer to the targetsubcooling value. If the refrigerant charge level was previouslyincreased and the subcooling value is now closer to the targetsubcooling value, controller 105 may continue to increase therefrigerant charge level. Likewise, if the refrigerant charge level waspreviously decreased and the subcooling value is now closer to thetarget subcooling value, controller 105 may continue to decrease therefrigerant charge level. However, if the refrigerant charge level waspreviously increased and the subcooling value is now further from thetarget subcooling value, controller 105 may begin decreasing therefrigerant charge level. If the refrigerant charge level was previouslydecreased and the subcooling value is now further from the targetsubcooling value, controller 105 may begin increasing the refrigerantcharge level.

To assist controller 105 in determining whether to increase or decreasethe refrigerant charge level, a liquidity sensor may be added to theliquid line. The liquidity sensor may be an optical or turbidity sensorwhich looks for bubbles through a side glass in the liquid line. Theabsence of bubbles indicates there is sufficient refrigerant chargelevel in the liquid line. Thus, if the liquidity sensor finds therefrigerant is sufficiently free of bubbles, controller 105 may alwaysdecrease the refrigerant charge level.

With reference to FIG. 6, depicted is an alternate method 600 whichcontroller 105 may perform to control the refrigerant charge level.Method 600 uses EER (Energy Efficiency Ratio) to determine whether therefrigerant charge level should be changed. EER is the ratio of energyexpended to the amount of heating or cooling performed. The EER is anindicator of the effectiveness of the system 100 or 300. The higher theEER, the more efficiently the system is operating.

In a compressor driven by an inverter, the amount of energy beingexpended can be obtained from the inverter driving the compressor. In acompressor not driven by an inverter, the amount of energy beingexpended can be measured from compressor current, compressor voltage,and phase angle at the compressor. The amount of heating or coolingperformed is measured by a heat transferring capacity of the HVACsystem. The heat transferring capacity may be the sensible capacity ofthe system, regardless of whether the system is heating or cooling. Ifthe system is cooling, the heat transferring capacity may alternately bethe latent capacity of the system, or the total of the sensible andlatent capacities of the system.

The sensible capacity may be expressed as the product of indoor airflowrate, a constant, and rise in air temperature. The sensible capacity maytherefore be calculated from the indoor airflow, return air temperature,and supply air temperature. The return air is the volume of air returnedto indoor unit 102 from the structure. The supply air is the volume ofair passed over indoor heat exchanger 118 and discharged to thestructure. Indoor unit 102 may have a return air temperature sensorwhere it receives the return air and a supply air temperature sensorwhere it discharges the supply air.

The latent capacity may be predicted from lab test data and presentconditions, such as indoor temperature, humidity, and indoor airflow.Alternately, latent capacity may be predicted from the rate ofcondensate (water vapor that is condensed on the surface of theevaporator).

Controller 105 may have a memory which stores target EERs for a givenload on the HVAC system. The target EERs represent an ideal EER when therefrigerant charge level is optimized for a given load. These targetEERs may be determined during testing or simulation of the HVAC system.

At 602, controller 105 may measure the energy used by the compressor andsensible capacity of the HVAC system. At 604, controller 105 maycalculate the EER from the energy used and sensible capacity. At 606,controller 105 may compare the EER to the target EER for the presentoperating load. At 608, controller 105 may operate valves 128A, 128B,and 128C on charge control apparatus 101 or charge control apparatus 103to adjust the refrigerant charge level. 608 may be performed identicallyto 508, except with the difference between the EER and target EER usedin place of the difference between the subcooling value and targetsubcooling value.

The size of reservoirs 126 and 302 may vary depending on the particularHVAC system. A reservoir should be large enough to accommodate thedifference between the largest and smallest optimal refrigerant chargelevels for the different operating loads of the system.

It is noted that the embodiments disclosed are illustrative rather thanlimiting in nature and that a wide range of variations, modifications,changes, and substitutions are contemplated in the foregoing disclosureand, in some instances, some features of the present invention may beemployed without a corresponding use of the other features. Many suchvariations and modifications may be considered desirable by thoseskilled in the art based upon a review of the foregoing description ofvarious embodiments.

We claim:
 1. An apparatus for adjusting refrigerant charge level, theapparatus comprising: a reservoir; a reservoir line connecting thereservoir and a liquid line, the liquid line connecting an indoor heatexchanger and an outdoor heat exchanger, the reservoir line comprising aconnection to the liquid line; a reservoir valve on the reservoir line;and one or more side valves on the liquid line.
 2. The apparatus ofclaim 1, wherein the one or more side valves comprises an indoor sidevalve on the liquid line between the indoor heat exchanger and theconnection to the reservoir line.
 3. The apparatus of claim 1, whereinthe one or more side valves comprises an outdoor side valve on theliquid line between the outdoor heat exchanger and the connection to thereservoir line.
 4. The apparatus of claim 1, wherein the one or moreside valves comprises: an indoor side valve on the liquid line betweenthe indoor heat exchanger and the connection to the reservoir line; andan outdoor side valve on the liquid line between the outdoor heatexchanger and the connection to the reservoir line.
 5. The apparatus ofclaim 1, wherein the reservoir valve and the one or more side valveseach comprise a solenoid valve.
 6. The apparatus of claim 1, wherein thereservoir is above the liquid line.
 7. The apparatus of claim 1,wherein: a suction line passes through the reservoir; and the suctionline is connected to a compressor.
 8. A method for adjusting refrigerantcharge level, the method comprising: calculating an indicator ofeffectiveness of a refrigerant-using system; comparing the indicator toa target indicator of effectiveness; and adjusting a refrigerant chargelevel to reduce the difference between the indicator and the targetindicator.
 9. The method of claim 8, wherein adjusting the refrigerantcharge level comprises opening or closing a solenoid valve.
 10. Themethod of claim 8, wherein: the indicator comprises a subcooling value;the target indicator comprises a subcooling value; and calculating theindicator comprises measuring a liquid temperature and liquid pressure.11. The method of claim 8, wherein: the indicator comprises an energyefficiency ratio; the target indicator comprises an energy efficiencyratio; and calculating the indicator comprises: measuring an energyusage of a compressor; and measuring a heat transferring capacity of thesystem.
 12. The method of claim 11, wherein the heat transferringcapacity comprises a sensible capacity of the system.
 13. The method ofclaim 12, wherein measuring the sensible capacity comprises measuring anindoor airflow, a return air temperature, and a supply air temperature.14. The method of claim 11, wherein the heat transferring capacitycomprises a latent capacity of the system.
 15. The method of claim 11,wherein the heat transferring capacity comprises a total of: a sensiblecapacity of the system; and a latent capacity of the system.